Fix weird planning issues when already fully migrated.

django/db/migrations/executor.py

@@ -33,10 +33,15 @@ class MigrationExecutor(object):
             # If the migration is already applied, do backwards mode,
             # otherwise do forwards mode.
             elif target in applied:
-                for migration in self.loader.graph.backwards_plan(target)[:-1]:
-                    if migration in applied:
-                        plan.append((self.loader.graph.nodes[migration], True))
-                        applied.remove(migration)
+                backwards_plan = self.loader.graph.backwards_plan(target)[:-1]
+                # We only do this if the migration is not the most recent one
+                # in its app - that is, another migration with the same app
+                # label is in the backwards plan
+                if any(node[0] == target[0] for node in backwards_plan):
+                    for migration in backwards_plan:
+                        if migration in applied:
+                            plan.append((self.loader.graph.nodes[migration], True))
+                            applied.remove(migration)
             else:
                 for migration in self.loader.graph.forwards_plan(target):
                     if migration not in applied:

tests/migrations/test_executor.py

@@ -12,7 +12,7 @@ class ExecutorTests(TransactionTestCase):
     test failures first, as they may be propagating into here.
     """
 
-    available_apps = ["migrations"]
+    available_apps = ["migrations", "django.contrib.sessions"]
 
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations"})
     def test_run(self):
@@ -38,3 +38,35 @@ class ExecutorTests(TransactionTestCase):
         # Are the tables there now?
         self.assertIn("migrations_author", connection.introspection.get_table_list(connection.cursor()))
         self.assertIn("migrations_book", connection.introspection.get_table_list(connection.cursor()))
+
+    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations", "sessions": "migrations.test_migrations_2"})
+    def test_empty_plan(self):
+        """
+        Tests that re-planning a full migration of a fully-migrated set doesn't
+        perform spurious unmigrations and remigrations.
+
+        There was previously a bug where the executor just always performed the
+        backwards plan for applied migrations - which even for the most recent
+        migration in an app, might include other, dependent apps, and these
+        were being unmigrated.
+        """
+        # Make the initial plan, check it
+        # We use 'sessions' here as the second app as it's always present
+        # in INSTALLED_APPS, so we can happily assign it test migrations.
+        executor = MigrationExecutor(connection)
+        plan = executor.migration_plan([("migrations", "0002_second"), ("sessions", "0001_initial")])
+        self.assertEqual(
+            plan,
+            [
+                (executor.loader.graph.nodes["migrations", "0001_initial"], False),
+                (executor.loader.graph.nodes["migrations", "0002_second"], False),
+                (executor.loader.graph.nodes["sessions", "0001_initial"], False),
+            ],
+        )
+        # Fake-apply all migrations
+        executor.migrate([("migrations", "0002_second"), ("sessions", "0001_initial")], fake=True)
+        # Now plan a second time and make sure it's empty
+        plan = executor.migration_plan([("migrations", "0002_second"), ("sessions", "0001_initial")])
+        self.assertEqual(plan, [])
+        # Erase all the fake records
+        executor.recorder.flush()

tests/migrations/test_migrations_2/0001_initial.py

@@ -0,0 +1,21 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("migrations", "0002_second")]
+    
+    operations = [
+
+        migrations.CreateModel(
+            "OtherAuthor",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("name", models.CharField(max_length=255)),
+                ("slug", models.SlugField(null=True)),
+                ("age", models.IntegerField(default=0)),
+                ("silly_field", models.BooleanField(default=False)),
+            ],
+        ),
+        
+    ]